Ionic bioaccumulation

Hilmy AM, El Domiaty NA, Daabees AY, Moussa FI. 1987. Short-term effects of mercury on survival, behaviour, bioaccumulation and ionic pattern in the catfish Clarias lazera). Comp Biochem Physiol C 87 303-308. 

Mercury (Hg) can occur in a large number of physical and chemical forms with a variety of properties, thus determining complex distribution, bioavailability, and toxicity patterns [1]. The most important chemical forms are elemental Hg (Hg°), ionic Hg (Hg2+ and Hg22+), and alkylmercury compounds. Because of their capability to permeate through biological membranes and to bioaccumulate and to biomagnificate through the trophic chain, alkylmercury compounds are the most toxic mercury species found in the aquatic environment [2]. 

Historically, organic environmental pollutants were hydrophobic, often persistent, neutral compounds. As a consequence, these substances were readily sorbed by particles and soluble in lipids. In modern times, efforts have been made to make xenobiotics more hydrophilic - often by including ionisable substituents. Presumably, these functional groups would render the compound less bioaccumulative. In particular, many pesticides and pharmaceuticals contain acidic or basic functions. However, studies on the fate and effect of organic environmental pollutants focus mainly on the neutral species [1], In the past, uptake into cells and sorption to biological membranes were often assumed to be only dependent on the neutral species. More recent studies that are reviewed in this chapter show that the ionic organic species play a role both for toxic effects and sorption of compounds to membranes. 

In a study of the bioaccumulation of metals as colloid complexes and free ions by the marine brown shrimp, Penaeus aztecus [29] the colloids were isolated and concentrated from water obtained from Dickinson Bayou, an inlet of Galveston Bay, Texas, using various filtration and ultrafiltration systems equipped with a spiral-wound 1 kDa cutoff cartridge. The total colloidal organic carbon in the concentrate was found to be 78 lmgdm 3. The shrimps were exposed to metals (Mn, Fe, Co, Zn, Cd, Ag, Sn, Ba and Hg) as radiolabelled colloid complexes, and free-ionic radiotracers using ultrafiltered seawater without radiotracers as controls. The experiments were designed so that the animals were exposed to environmentally realistic metal and colloid concentrations. 

Carvalho, R. A., Benfield, M. C. and Santschi, P. H. (1999). Comparative bioaccumulation studies of colloidally complexed and free-ionic heavy metals in juvenile brown shrimp Penaeus aztecus (Crustacea Decapoda Penaeidae), Limnol. Ocea-nogr., 44, 403 -14. 

In addition, bioaccumulation in aquatic organisms has been studied, again mainly for LAS [62], the non-ionic surfactants and their degradation products (see Chapter 7.2). A close relation with hydrophobicity can be drawn from studies performed with pure homologues of LAS. 

Finally, it is interesting to note that brominated derivatives of NP included in the present study, estrogenic in the E-Screen bioassay, were proposed to be fat-soluble [30], In fact, as non-ionic surfactants, these molecules have a hydrophilic and a hydrophobic part [31]. The presence of these brominated compounds in fat tissue of humans or animals has yet to be demonstrated. However, if they do bioaccumulate in adipose tissue, as their fat solubility suggests, they may account for the xenoestrogen burden alongside organohalogenated compounds. 

Poldoski, 3.E., 1979. Cadmium bioaccumulation assays. Their relationship to various ionic equilibria in Lake Superior water. Environ. Sci. Technol., 13 701-706. 

Physiological Models for chemical bioaccumulation in fish are based on the same mass balance equations as the kinetic models for bioaccumulation, but the rate constants and chemical fluxes that quantify the rates of uptake and elimination of the substance are derived from Kow and a set of physiological parameters. The most well known model in this category is the FGETS (Food and Gill Exchange of Toxic Substances) model Barber et al. (1988, 1991) developed. This is a FORTRAN simulation model that predicts dynamics of a fish s whole body concentration of non-ionic, nonmetabolized, organic chemicals absorbed from the water only, or from water and food jointly. 

Methyl mercury is of much greater concern when health effects are considered, as it is much more toxic than ionic mercury or free mercury. Methyl mercury is also much more likely to be bioaccumulated, leading to serious contaminations, especially of fish. The speciation for mercury can be accomplished by derivatizing the methyl mercury and Hg2+ with sodium tetraethylborate, NaBEt4. The volatile MeHgEt, from methyl mercury, and HgEt2, from Hg2+, species formed are purged from the sample solution and separated in a GC column. An atomic emission spectrometer is used as a detector. 

No experimental data regarding the bioconcentration potential of DNOC in aquatic organisms were located. Based on an estimated bioconcentration factor (BCF) of 40 (Kenaga 1980), the bioconcentration of DNOC in aquatic organisms may not be significant however, based on an estimated log octanol/water partition coefficient [log(K°w)j value of 2.85, DNOC may bioaccumulate in aquatic organisms (Loehr and Krishnamoorthy 1988). Given that DNOC exists predominantly in ionic forms in most natural waters (pH 5-9) and that the compound is markedly toxic to fish, bioconcentration is not expected to be important (EPA 1979). 

The octanol-water partition coefficient for surfactants can not be determined using the shake-flask or slow stirring method because of the formation of emulsions. In addition, the surfactant molecules will exist in the water phase almost exclusively as ions, whereas they will have to pair with a counter-ion in order to be dissolved in octanol. Therefore, experimental determination of K w does not characterize the partition of ionic surfactants (Tolls, 1998). On the other hand, it has been shown that the bioconcentration of anionic and non-ionic surfactants increases with increasing lipophilicity (Tolls, 1998). Tolls (1998) showed that for some surfactants, an estimated log Kow value using LOGKOW could represent the bioaccumulation potential however, for other surfactants some correction to the estimated log Kow value using the method of Roberts (1989) was required. These results illustrate that the quality of the relationship between log Kow estimates and bioconcentration depends on the class and specific type of surfactants involved. Therefore, the classification of the bioconcentration potential based on log Kow values should be used with caution. 

The surface sediments are considered as the most probable place in which one part of the inorganic ionic mercury (Hg2+) is converted to monomethylmercury (MeHg+) subsequently bioaccumulated in the aquatic... 

PROBABLE FATE photolysis breakdown of atmospheric dimethyl mercury is of slight importance oxidation/reduction oxidation of metallic mercury forms ionic mercury (later adsorbed), reduction forms HgS precipitate hydrolysis not an important process volatilization metallic Hg, methylated Hg, and adsorbed Hg all volatilizable sorption Hg is adsorbed by most particles, buried in sediment, and reduced to HgS biological processes bioaccumulated by all organisms and readily methylated metabolically... 

A new type of ionic PAGs based on imides and methides has recently been proposed by 3M. The notable characteristic of these PAGs is that they do not contain perfluorooctyl sulfonates and related derivatives, which have recently been identified by the U.S. Environmental Protection Agency and environmental regulatory agencies in other countries to pose health hazards, as they are biopersis-tent and toxic, and bioaccumulate in mammalian tissues. 

As outlined in the previous paragraphs, the most important factor contributing to the sorption, desorption, toxicity and bioaccumulation of ionic liquids... 

Toxicides, biodegradation, and bioaccumulation are not yet sufficiently known for most of the Ionic Liquids. As a consequence. 

As discussed above, many ionic liquid structures seem to be persistent, or at least are only slowly degradable in the environment. This tendency of persistency, combined with the mostly relatively high hydrophobicity of ionic liquid cations and anions, bears the hazard of bioaccumulation. Thus, we suggest that a greater research effort be made to elucidate the bioaccumulation potential of ionic liquids and the underl3dng mechanisms involved. Since ionic liquids can be both hydrophobic and ionic, special mechanisms and distribution effects may come into play in their bioaccumulation behaviour. And, as already mentioned above, the possibly high persistency and slow degradation rates require more and detailed chronic effect studies in order to complete the hazard picture of at least the most frequently used ionic liquid structures. 

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